Method of forming in situ gelled anode

ABSTRACT

A method for forming a gelled anode in which a powdered anode metal is mixed with means for maintaining a uniform mixture of the powdered metal and a gelling agent, the gelling agent is added, and the mixture is fed into an aqueous electrolyte in the cell whereupon a gel is formed in situ.

FIELD OF THE INVENTION

The present invention relates to a method of in situ gelling of theanode with electrolyte of electrochemical cells and a gelling agent, andmore particularly to the gelling of the anode and electrolyte in situthrough the use of gelling agents which comprise materials capable ofabsorbing water and means for maintaining a uniform mixture of the anodeand the gelling agent during in situ gelling.

BACKGROUND OF THE INVENTION

A thickened or gelled electrolyte is less likely to leak from anelectrochemical cell. Many materials have been used as thickening orgelling agents in the past. Such agents include soluble thickeners,insoluble absorbents such as starch, various cellulose type materialssuch as methyl cellulose, and some synthetic polymers.

A recurring problem with agents used heretofore has been that onstanding or during cell discharge, liquid separated from many of thethickened solutions or gels. The liquid was then able to leak out of thecells. Further, during the manufacture of the cells and before theaddition of the mixture to the cells, without constant agitation of themixture, separation often occurred. This in turn resulted in theinaccurate addition of electrolyte due to the random ratios of liquidand gel being added to the cells. The inaccurate addition of electrolyteto the cells in many cases produced poor cells.

Increasing the quantity of agent added to the electrolyte sometimesreduced or prevented this separation, but it also decreased the volumeand weight of active material in the cells. The greater quantity ofgelling agent also generally decreased the ionic conductivity of theelectrolyte which in turn increased the internal resistance of thecells.

Another drawback to the use of some known agents is that they weresubject to chemical attack by the strong basic electrolyte solutionsemployed in alkaline type cells or the acid electrolyte employed inother types of cells. Similarly, some agents also attacked or wereattacked by the various components of the cell. The decompositionproducts resulting from these reactions adversely affected theperformance of many cells.

In some cells, a thickener was also added to the anode and/or cathode.Generally, the thickener was similar to that used for the electrolyte.The electrodes were gelled for many reasons depending on the type ofcell involved and the results desired.

The use of water-insoluble or water absorbable agents such as thosedisclosed in U.S. patent application Ser. No. 106,996 filed on even dateherewith have not only reduced the above enumerated problems but alsounexpectedly improved the discharge capacity of the anode. These agents,even though improving the discharge capacity of cells made therewith donot consistantly form highly uniform gels.

THE INVENTION

In the manufacture of an electrochemical cell comprises of an anode, anaqueous alkaline electrolyte solution, a separator and a cathode,wherein the anode comprises an intimate gelled mixture of a powderedmetal, a portion of the aqueous alkaline electrolyte solution and anagent capable of gelling the mixture, where the agent comprises amaterial capable of absorbing water, a method has now been discovered bywhich the anode and a portion of the aqueous electrolyte solution can begelled in situ. This novel method results in the metal powder beingsubstantially uniformly distributed throughout the gel. The methodcomprises the steps of partially forming a cell in a conventional mannerwith a cathode and a separator in a container. Then electrolyte isadded. Separately, a mixture is made by admixing an active anode metalin powdered form with a means, described more fully below, formaintaining a uniform mixture of the powdered metal and a gelling agentduring in situ gelling. Then the gelling agent is added thereto andthoroughly admixed therewith. This mixture is dispensed into the aqueouselectrolyte solution in the cell container whereby a uniform gel isformed. The means for maintaining uniformity is a material which willhold the mixture of powdered metal and gelling agent together while thegelling agent is absorbing water, thereby forming a gel in situ, inwhich gel the metal particles are uniformly distributed.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred means for maintaining homogeneity are liquids which canform a thin adherent film or layer on the surface of the metal particlesand/or the particles of gelling agent. The preferred means should beable to form an effective layer of minimum thickness so that very littleis required. The preferred means should also be water soluble ormiscible so that it does not prevent the aqueous electrolyte solutionfrom contacting the surface of the metal particles. Water insolublemeans could form a passivating layer which might interfere with propercell functioning and discharge. In a most preferred embodiment theadditive also imparts some corrosion resistance to the metal particles,thereby reducing cell gassing.

Presently preferred means for maintaining uniformity include liquidpolyhydric alcohols. The trihydric alcohols and glycols are preferredwith glycerol (glycerine) being most preferred. Most of the polyhydricalcohols have lubricating properties which aid in the substantiallyhomogeneous dispersion of the gelling agent through the powdered metal.They also have some adhesive properties which can aid in holding theparticles of metal and gelling agent together when dispersed into theelectrolyte solution and during the gelling of the powdered metal andelectrolyte solution.

Examples of glycols include ethylene glycol, diethylene glycol,triethylene glycol, polyethylene glycols, propylene glycol, trimethyleneglycol, and tetramethylene glycol. Other liquid polyhydric alcoholshaving chemical and physical properties similar to glycerol are wellknown in the art and also useful in the present invention.

The effective quantity of means for maintaining uniformity useful in thepresent invention is unexpectedly quite small, varying from less than0.1 percent to about 2.5 percent of the weight of the powdered anodemetal. The preferred amount is from about 0.1 to 0.4 percent with about0.25 percent of the weight of the anode metal being most preferred. Useof these small quantities of added material does not significantlydecrease the energy density of the cell.

After the powdered metal, the means for maintaining homogeneity, and thegelling agents are substantially homogeneously admixed, a small quantityof water can advantageously be dispersed therein. The effective quantityof water added is also small, and can vary from less than 0.3 percent toabout 1 percent of the anode metal weight. The preferred quantity ofwater used varies, depending on the chemical means for maintaininguniformity. The quantity of water used with the preferred agent,glycerine, is about 0.8 percent of the powdered metal weight.

The gelling agents of the present invention have been disclosed incopending U.S. patent application Ser. No. 106,996, filed on even dateherewith which is incorporated herein by reference, and arewater-insoluble salts of aqueous, alkali saponified material comprisinga gelatinized starch backbone having at least one water soluble sidechain grafted thereon. The gelatinized starch and saponified side chainsare preferably in molar ratios of from about 1:1 to 1:19. These graftedgelatinized starch materials have the ability to absorb or gel a greatquantity, up to more than 1,000 times their own weight, of water. Adetailed description and various methods of manufacture of two suchpreferred agents, having a grafted acrylonitrile or methacrylonitrileside chain are found in U.S. Pat. Nos. 3,935,009, 3,981,100 and3,997,484 (all Weaver et al.), the entire disclosures of all of whichare incorporated herein by reference. Other similar gelling agents aredisclosed in U.S. Pat. No. 3,661,815 (Smith). Similar ungelatinizedcarbohydrate backbone agents are disclosed in U.S. Pat. No. 3,425,971(Gugliemelli et al.). These further references are also incorporatedherein by reference.

Unexpectedly, the use of the process disclosed herein increases thepractical discharge capacity of the anode even beyond that of thecopending application Ser. No. 106,996 referred to above. In a preferredcell using amalgamated zinc as the anode metal and wherein the gel isformed in situ, upwards of a thirty percent increase in practical anodedischarge capacity at high rate is found to occur when the presentprocess is used as compared to prior art cells. It is believed that thisis a result of the greater homogeneity of the anode and the increasedspace between the individual particles of zinc powder in the gelledanode-electrolyte mixture within the cell as compared to the spacebetween the particles when an equal weight of previously used zincpowder electrolyte mixture is thickened or gelled in a conventionalmanner.

Electrochemical cells having an aqueous electrolyte are subject tointernal corrosion and the generation of hydrogen gas. Both areundesirable. Unexpectedly, it has been found that the use of the presentinvention significantly reduces the production of gas.

In the present invention, the anode is a gelled mixture of theelectrolyte solution and a metal in a particulate form. The metal usefulin the anode of the present invention can be any metal generally used incells having an aqueous electrolyte. Such metals can include aluminum,cadmium, calcium, copper, indium, iron, lead, magnesium, manganese,mercury, nickel, tin, zinc and other metals well known in the art, usedeither alone or in various combinations.

In the preferred cell, the anode metal comprises powdered amalgamatedzinc. The preferred anode metal powder is on the order of from about0.03 to 0.9 millimeter in diameter. The most preferred size of powder tobe used depends on many factors and it can be readily determined by oneskilled in the art.

The electrolyte solutions which can be gelled by the agents of thepresent invention include those aqueous electrolyte solutions useful inelectrochemical cells. In the preferred embodiments of the presentinvention alkaline electrolyte solutions are employed. These include,but are not limited to, hydroxides of alkali and alkaline earth metals.Sodium and/or potassium hydroxide are the most commonly used alkalineelectrolytes. The agents of the present invention, being stable towardsacids, can also be used with acid electrolyte solutions, for example,those employed in the well known zinc-carbon or lead acid batterysystems.

The agents and chemical means for maintaining uniformity of the presentinvention can apparently be used with all cathodes heretofore used inaqueous electrochemical cells. These cathodes include, but are notlimited to metal oxides, such as cadmium oxide and hydroxide, mercuricoxide, lead oxide, manganese dioxide, nickel oxide and silver oxide.

A separator can be employed in the present invention between the gelledanode-electrolyte mixture, and cathode. Such separators are similar tothose well known in the art and used in various aqueous electrochemicalcells. Useful separator materials include, but are not limited, toporous cellulose, plastic and glass materials.

The advantages and efficacy of the present invention are illustrated inthe following examples. In the examples and claims all percentages,unless otherwise indicated, are by weight.

EXAMPLE 1

A mixture is made by combining 500 kilograms of amalgamated zinc powder,which is 93 percent zinc and 7 percent mercury with 1.2 kilograms ofglycerine (glycerol), then 4 kilograms of water are added, and then 19kilograms of a powdered gelling agent sold under the trademark SGP145*by Henkel Corporation, Minneapolis, Minn. are added.

Conventional cell cans are prepared each having a cathode therein ofabout 40 grams of manganese dioxide, 5 grams of graphite and 5 grams ofa 9 N potassium hydroxide solution. A conventional cellulose typeseparator is added. Then 20 milliliters of an aqueous electrolytesolution, comprising about 35 percent by weight of potassium hydroxideand two percent zinc oxide is added. The electrolyte wets the separatorand cathode. About 17 grams of the amalgamated zinc, glycerine, waterand gelling agent mixture is then added to the cell can. This mixturesinks through the electrolyte and a homogeneous gel then forms. Themanufacture of the cell is completed in a conventional manner.

On standing, liquid does not separate out from the gel, nor does theamalgamated zinc settle out. The density of the gelled mixture is lowerthan that of a mixture of similar composition using conventionalthickeners.

On discharge through a 2.25 ohm resistor to a 0.8 volt cutoff the cellexhibits electrical characteristics similar to those cells made withconventional thickeners such as sodium carboxymethyl cellulose.Unexpectedly, the cell exhibits a higher discharge capacity of aboutthirty percent, discharging for 17 hours, as opposed to 13 hours for thecell made with sodium carboxymethyl cellulose.

Upon storage of these cells for a period of one to three months at roomtemperature and at temperatures of 0° C., 45° C., and 75° C., much lesshydrogen evolution is evident on storage than with cells not using theSGP 145 agent and glycerine. It is evident that the reduction in gassingpermits a reduction in the quantity of mercury normally used in the cellto reduce gassing, because with a system less prone to gassing, lessmercury would be needed to achieve any desired rate of gassing.

EXAMPLE 2

A mixture, as in Example 1, is prepared by combining 490 kilograms ofamalgamated zinc anode powder, which is 93 percent zinc and 7 percentmercury, with 1.2 kilograms glycerine (glycerol) then with 4 kilogramsof water and then with 40 kilograms of an alkali metal carboxylate saltof a saponified starch polyacrylonitrile grafted copolymer made inaccordance with the teachings of Example I (copolymer A) of U.S. Pat.No. 3,425,971. The resulting mixture is added to cells as in Example 1.

The resultant gel is similar to and has properties similar to the gelproduced in Example 1 but requires more agent to gel the same volume ofelectrolyte solution used in Example 1. On discharge the cell haselectrical properties similar to the cell of Example 1.

EXAMPLES 3-8

Cells are made in accordance with the procedures described in Example 1but using agents having methyl methacrylate, acrylamide, acrylic acid,N-vinyl-2 pyrrolidone, alginic acid, and gluconic acid, respectively, asthe side chain grafted onto a gelatinize starch backbone. The graftingis done in a method similar to that shown in the disclosures of Weaveret al. The side chains are ionized in a manner well known in the art.The cells are tested similarly to that in Example 1 and are found tohave substantially similar efficacy.

EXAMPLES 9-15

Cells are made in accordance with the procedure described in Example 1but using ethylene glycol, diethylene glycol, triethylene glycol,polyethylene glycol, propylene glycol, trimethylene glycol, andtetramethylene glycol respectively as the chemical means for maintaininguniformity. The cells are tested similarly to those of Example 1 and arefound to have significantly improved discharge capacity when compared toprior art cells.

EXAMPLE 16

Cells are made in accordance with the procedure described in Example 1but in which about 90 grams of silver oxide is used to replace themanganese dioxide in the cathode. The cells are tested similarly and arefound to have equal or better discharge characteristics than similarcells which are made using prior art procedures.

EXAMPLE 17

Cells are made in accordance with the procedure described in Example 1but in which about 130 grams of mercuric oxide is used to replace themanganese dioxide in the cathode. The cells are tested similarly and arefound to have equal or better discharge characteristics than cells whichare made using prior art gelled anodes.

The preceeding examples are for illustrative purposes only. It isunderstood that variations and changes can be made without departingfrom the spirit and scope of the present invention as defined in thefollowing claims.

What is claimed is:
 1. A method for forming a cell having a substantially homogeneously gelled anode comprising the steps of mixing a powdered anode metal with a gelling agent, and with liquid means other than the gelling agent for maintaining, during subsequent gelling, a homogeneous mixture of the powdered metal, said gelling agent and a liquid, said gelling agent being capable of gelling the powdered anode metal when in the presence of an aqueous electrolyte solution; and then dispensing the resulting ungelled mixture into an electrolyte solution in an electrochemical cell, whereby the substantially homogeneously gelled anode is formed in situ.
 2. The method of claim 1 wherein said means for maintaining homogeneity is glycerine.
 3. The method of claim 1 wherein the means for maintaining homogeneity is mixed with the powdered anode metal prior to admixture with the gelling agent.
 4. The method of claim 1 wherein said resulting mixture also contains water.
 5. The method of claim 4 wherein said water is present in an amount up to about one percent.
 6. The method of claim 1 wherein the means for maintaining homogeneity comprises a liquid polyhydric alcohol.
 7. The method of claim 6 wherein the liquid polyhydric alcohol is selected from the group consisting of glycerine, ethylene glycol, diethylene glycol, triethylene glycol, liquid polyethylene glycols, propylene glycol, trimethylene glycol, and tetramethylene glycol.
 8. The method of claim 1 wherein the means for maintaining homogeneity is present in an effective amount up to about 2.5 percent of the weight of the powdered anode metal.
 9. The method of claim 8 wherein the means is glycerine and is present in an amount of from about 0.1 percent to about 0.4 percent of the weight of the powdered anode metal.
 10. The method of claim 1 wherein the gelling agent comprises a carbohydrate backbone having a water soluble side chain grafted thereon.
 11. A method for forming a cell having a substantially homogeneously gelled anode comprising the steps of mixing a powdered anode metal with a gelling agent, and with a polyhydric alcohol for maintaining, during subsequent gelling, a homogeneous mixture of the powdered metal, said gelling agent and a liquid, said gelling agent being capable of gelling the powdered anode metal when in the presence of an aqueous electrolyte solution; and then dispensing the resulting ungelled mixture into an electrolyte solution in an electrochemical cell, whereby the substantially homogeneously gelled anode is formed in situ. 